Capacitors are devices that store electrical energy on a dielectric material between two conductive electrodes, usually metal. Their architecture has followed a path that started as a metal sleeve on the outside and inside of a glass jar (called a Leyden Jar). Today, materials with very high dielectric constants are used in geometries that maximize the area of the electrodes while at the same time providing dielectric thicknesses that result in high breakdown voltages. The combination of high dielectric constant, high voltage, and high electrode surface area leads to an energy storage capability with very high specific energy (energy per unit volume).
Recently it has been reported (U.S. Pat. No. 7,023,687, incorporated herein by reference) that very small particles of Barium Titanate (BaTiO3) coated with a layer of Alumina (Al2O3) can have very high dielectric constants (values near 20,000) when they are preferentially oriented by an external electric field. The cited inventors have chosen to fill the gaps between the particles with a plastic (the chosen plastic is PET) so as to increase the breakdown voltage of the dielectric layer and to hold the particles in place. This solution uses powdered metal (Aluminum) to create the conducting electrodes. In order to access the increased dielectric constant of the BaTiO3 material, a high voltage is applied to the matrix between the electrodes while the system is held at a temperature high enough for the PET to be soft to allow the BaTiO3 particles to orient. The layer of BaTiO3 particles needs to be relatively thin in order to have a high enough field strength to cause the preferential orientation. One problem has been the fact that most dielectric materials used in this application have a breakdown voltage that is lower than the voltage needed for the orientation effect. The inventors reported that the breakdown voltage can be held just above the required value if the BaTiO3 particles are densely packed in the PET matrix of the dielectric layer.
Even more recent is an application in Japan by the Japanese Aerospace Exploration Agency and TDK (US patent application pub. No. 2011/0059838 for Dielectric Ceramic Composition, incorporated herein by reference). The material in this application is also BaTiO3 made with an additional small amount of Lanthanum, Cerium, Vanadium, or Bismuth. This addition and processing method provides a hexagonal form of BaTiO3 which has a measured dielectric constant of 160,000.
However, there are problems with practical devices based on this architecture using PET. If the BaTiO3 coated particles have a lower density in some areas of the plastic matrix, then a lower breakdown voltage will be experienced in that specific area. This results in local current breakdown with high heating and subsequent destruction of the device. This susceptibility to catastrophic breakdown is further exacerbated by a residual heating effect in the powdered metal electrodes due to a resistance from metal particle to particle, and to a residual heating effect in the BaTiO3 material due to charging and discharging. While this effect may be small, the temperature effect is evident in the charge/discharge operation and important for the life and reliability of the device. These concerns can therefore severely limit the maximum allowed applied voltage to the device and thereby limit the amount of energy storage, limiting the utility of the resulting device.